The present invention relates to an electronic shutter for an electronic video camera, especially for a still video camera using an interlace scanning type image sensor.
As an imaging device for an electronic still camera, a CCD-type or MOS-type solid state imaging device such as an image sensor is used. Many of the image sensors adopt a frame interlace scanning system, whereby signal charges of one frame image are read out separately for each field, that is, an odd field and an even field. Using the frame interlace scanning system makes it possible to read the image not only in a frame storage mode but also in a field storage mode, so-called a pseudo frame storage mode which is where the signal charges of an odd field are mixed with those of an even field so as to improve the S/N ratio even when the subject brightness is low.
It is well-known in the art that when using such an image sensor in a camera, because the charge storage time of the image sensor is equivalent to the exposure time, a mechanical shutter can be omitted if only the charge storage time of the image sensor is controlled instead. The exposure control of the image sensor is performed such that upon a first field shift pulse unnecessary charges is swept away from the image sensor through a drain and, in that moment, the image sensor starts storing signal charges in an imaging portion for an appropriate time corresponding to the subject brightness, thereafter a second field shift pulse is input to transfer the signal charges at once to charge transfer channels of the image sensor, from which the signal charges are sequentially sent out of the image sensor in response to vertical transfer pulses and horizontal transfer pulses. The above charge storage time control is called electronic shutter control when exposure time control is performed.
Conventionally, the charge storage time is determined by selecting an optimum one of several predetermined time periods, for example, 1/60 sec., 1/125 sec., 1/250 sec., etc. In this type of exposure control method, however, it is difficult to adjust the exposure finely so that the above stepwise exposure control sometimes results in under-exposure or overexposure, this is conspicuous in an electronic still camera compared with a movie camera.
In order to solve this problem, it may be possible to make the increments of predetermined exposure times finer, but this solution will increase the number of control signal lines to be connected to an exposure control unit. As a result, the assembling of the circuit becomes complicated and the cost for a camera increases.